Vitamin D, or calciferol, is the general name for a collection of
steroid-like substances including vitamin D2, ergocalciferol
(fig.1) and vitamin D3, cholecalciferol (fig.2). Found only in
animal sources, vitamin D is fat soluble and can be manufactured by
the body on exposure to UV radiation.

In the body, the vitamin D is converted to its active hormone form
by the liver and the kidney. The calciferol is converted to
25-hydroxycholecalciferol, calcidiol (fig.3), and then metabolized
by the kidney to several active forms including
1,25-dihydroxycholecalciferol, calcitriol (fig.4).

The main function of vitamin D in the body is to maintain blood
levels of calcium and phosophorus. Vitamin D is essential for the
absorption of these substances from the small intestine, and also
for the mineralization process of the bones. The result of vitamin
D deficiency is rachitis (rickets) in children and osteomalacia in
adults.